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Reactions of (1-chlorovinyl)diphenylphosphine oxide with organometallic reagents
Teh&&,,,,
Letren. Vol. 34, No. 19. Pp. 31353138.1993
Priitcd in Grut Britain
Reactions of (l-ChlorovinyQdiphenylphosphine
Oxide
with Organometallic Reagents codimu car&nicdl&P,
kanccaco NH’, WihkWkP
Vito Fim,
F+l&ddd*,
K. Mkhd
and witdd
(‘) C.N.R., Centro di Stualiosulk Metodogie
Innovative di Sintesi Organiche. Dipartimento di Chimica, via Amendola 173,70126 Bari, Italy.
(b) Polish Academy of Science, Centre of hfokcular and Macromokcular Studies, 90-363 LU?, Sienkiewicza 112. Poland
We have recently reported the results of our studiis on the reactions of halovinyl aryl sulfoxides with organocuprates, organomagnesium
and organolithium compounds’~. In particularl. @‘,I-and Q-2-halovinyl
phenyl sulfoxides were found to tcact with alkyl- or phenylmagnesium bromides at room tempt?-,
leoding
to alkyl phenyl or diphenyl sulfoxides in good isolated yields (79-935). and a similar behaviour was found for 1-btomovinyl ~ulfoxides,~ according to Scheme 1. SCHEME 1 0
0
II
RMgBr
Ar-S-CH=CH-X
b
0
II ArS-R
4
THF
RMgBr
II Ar-S-C(Br)=CH2
THF R= alkyl. aryl
X= Cl, Br Optically active I-bromovinyl
and 2-btomovinyl
aryl sulfoxides ma&d
Grignard reagents (e.e.Z98%).2 Compkte inversion of configuration was establii
enantiospecifically
with
for the reaction with the
fast type of substrate. This stereochemical evidence is consistent with the hypothesis of an attack of the Grignard reagent at the sulphur atom of the halovinyl sulfoxide with a concomitant expulsion of a two c&on fragment, which then decomposes to give acetylene.’ These results
prompted
(1chlorovinyl)diphenylphosphii
us to investigate
the mactions
between
organometallic
teagents
and
oxide l? in order to compam the toactivity pattern between phosphorous 3135
3136
and sulphur compounds and to verify the possibility of a new synthetic approach to phosphine oxides, a class of compounds of special interest due to their versatile use: e.g. as complexing agentsP in the synthesis of natural producti and in applied chemistry.6 Diphenylphosphinyl chlorides’ and alkyl diphenylphosphinate# displacement
with
organomagnesium
reagents,
yielding
were reported to undetgo a nucleophilic
alkyldiphenylphosphine
oxides.
The
high
enantiospecificity of this process was applied to the synthesis of optically active phosphine oxides, starting from menthyl phosphinates,9 as a counterpart of the synthesis of optically active sulfoxides starting from menthyl sulfmate.tO Furthermore, it has been recently reported l1 that some phosphine oxides react with organolithium or Grignard reagents with a ligand exchange or dispropottionation, as it was reported for the analogous sulfoxides.‘2 We wish to report now that when (l-chlorovinyl)diphenylphosphine
oxide 1 was nxcted with Grignad
reagents, an attack of the organometallic compound at the phosphorous atom with consequent formation of phosphine oxides 210 was observed (see Table). Table. Reactions of (l-Chlorovinyl)diphenylphosphiie
Oxide 1 with Grignard Reagents
RMgBr
1 Ph2 P -R
Ph 2 ! - C(Cl)=CH 2’ THF, 25“C
1 bay : : : 7 f
Product
R Meb Et n-Pr c n-Bu c Ph z:g; p-Aniiyl 2-Np
210 Yield * 75
: z : 6
;:
s7 9 10
;: 78 38
a) Yields refer to pure isolated products; b) methylmagnesium chloride was used, c) reactions performed at CPCdid not show any signitlcant diierence on the products. As reported in the Table, good yields wete obtained when 1 reacted tith methylmagnesium chloride and with some aryl Grignard reagents (entries 1.5.6
and 8). If other alkylmagnesium reagents were used (entries
2-4). lower isolated yields of 3-5 wete obtained, due to the formation of minor quantities of several by-products, presumably originated by metalationS and further reaction of the alkyldiphenylphosphine
oxide
55 obtained in the fiit displacement reaction. Moderate yields were also obtained with o-tolylmagnesium and 2-naphthylmagnesium
bromide (entries 7
and 9). probably owing to steric masons. A similar argument was used to explain some unsuccessful reactions of hindered Grignard magenta with menthyl phosphinates.s9*13
3137
Gn the whole, the observations made with the Grignard reagents indicate a close analogy with the behaviour of the sulfoxides. The presented results confirm clearly the quality of the chlorovinyl group as a viable leaving group in heteroatom chemistry. It seems likely that, as in the sulfoxides, also in this case the fate of the two-carbon fragment is the decomposition to acetylene. A more complex pattern was observed when (1-chlorovinyl)diphenylphosphine cuprates in THP at 0°C (Scheme). Using a 1.51 cupmte:substrate
oxide 1 reacted with
ratio. compound
consumed. Among the examined organocuprates, n-Bu&uMgBr and Ph#tMgBr
1 was completely
gave the phosphine oxides
13 (isolated yield 49%) and 14 (yield 63%) as the main isolated products. Gas-chromatography/Mass Spectrometry analysis of both reaction mixtutes revealed minor quantities of the vinylphosphine oxides 11 tR= n-Bu. 35% of the tea&on ptoducts), 12 (R= Ph. 5%) and diphenylvinylphosphine
oxide 15 (less than 5% in
both reactions). Gn the other hand, n-BuaCuLi and n-Bu2Cu(CN)Liz caused mainly reduction of the substrate (i.e. formation of 15). while Ph&uLi gave the same products of Ph&uMgBr, although in lower yields (14 was
isolated in 47% yield) due to the formation of a greater amount of side-products.
Although a deeper
investigation would be necessary for the rationale of these observations, the formation of the above products could be tentatively explained according to the following Scheme.t4 SCHEME 2 0 II -
+
Ph+‘-C(Cl)CH2R
a-
0
II
PhzPCH=CH-R
(a)
1112
1) R&uM i Ph~PCH,CHR, 2) H+ W-14
R2CuM Ph2 P-C(Cl)=CH,
P G CUM II I Ph2 P-C=CH,
1 iI
-b
H+
00
ii
Ph, P-CH=CH, 16
11,13 M=MgBr
R = n-Bu
12,14 R=Ph
(a) The main pathway should be represented by an addition process leading to a species from which products 11-12 am originated.15 These undergo a further addition of cuprate to give 13 or 14. (b) The formation of vinyldiphenylphosphine
oxide 15 should be the result of a metal-halogen exchange followed by the final
neutralixation. In conclusion. the results of the present investigation suggest that synthetically useful teactions can be performed
reacting
I-halovinylphosphine
oxides with organometallic
reagents. Carbon-phosphorous
or
carbon-carbon bond can be formed depending upon the nature of the reagents, thus permitting to obtain different types of phosphine oxides from a common precursor.
3138
Rcactbn of (l-ChbrovinyI)diphenylphoephine Oxide 1 with Grignud Reagents, Synthesis of DIphenyhncthylp oxhk 2. A solution of 2.2 mmol of methyImagn&un chloride in THF was added dropwise to a stirred solution of 0.3 g (1.1 mmol) of 1 in 12 mL of THF, under Nz. After 1 h, the reaction mixture was quenched with a saturated aqueous NH&l solution and extracted three times with ethyl acetate. The combined organic extracts were washed with an aqueous solution of NaOH 1 N and with water, and dried (Na2SOd). Then the solvent was removed in vucuu. The residue was eluted through a very short column of basic aluminum oxide (eluent ethyl acetate). After vacuum distillation of the solvent, the residue was purified by crystallization (benzene@troleum ether, mp 109-l IOOC,lit.16 111.5-l 12 “c). Yield 75%. Rerction of (l-ChIorov@I~phenylpbo6phine Oxide 1 w&h Organocuprates. Synthesis of (2&dIphenykthylPiphenylpbasphk Oxide 14. A solution of 3.3 mmol of freshly prepamd phenylmagnesium bromide in ‘H-IF was added dropwise to a stirred slurry of 0.314 g (1.65 mmol) of CuI in 5 mL of THF at Ooc. After 10 min. a solution of 0.3 g (1.1 mmol) of 1 in 6 mL of THF was added dropwise. The reaction mixture was stirred at Ooc for 1 h and then was quenched with a saturated aqueous NH&l solution and extracted three times with ethyl acetate. The combined organic extracts were washed with water, dried (Na+O,) and the solvent was removed in VUCIW. (2,2-Diphenylethyl)diphenylphosphine oxide 14 was purified by crystallization (ethyl acetate; mp 221-222 “c). Yield 63%. ‘H NMR (500 MHz) 8 7.99-7.95 (m, 2 H), 7.58-7.03 (m, 16 H), 6.81-6.79 (m, 2 H), 3.64 (ddd. 5=3.5,7.6, 11.0 Hz, 1 H), 3.32 (ddd, .J= 5.7, 11.0, 14.1 Hz_ 1 H), 3.26 (ddd, 5=3.5,9.0, 14.1 Hz. 1 H). MS (70 ev): m/e (relative intensity) 382 (M.4). 202 (lOO), 180 (16), 165 (12). AckmMedgemcnt: Work supported in part by Miteto dell’Universita e della Ricerca Scientifica e Tecnologica, Rome and by the State Committee for Scientific Research, Poland. We thank dr. C. Bortone. dr. M.A. Prencipe and Mr. M. Koprowski for preliminary experiments. REFERENCES 1. Cardellicchio. C.; Fiidanese, V.; Naso, F. J. Org. Chem 1992,57,1718-1722. 2. CardelIicchio. C.; FianU, V.; Naso. F.; Scilimati, A. Terrahcdron Lerr.. 1992.57.5121-5124. 3. Pietrusiewicz, K.M.; Wisniewski, W.; Zablocka, M.; Tetruhcdron. 1989,#5,337-348 and references therein. 4. Dondi. S; Nardelli, M.; Pelizzi, C ; Pelizzi. G; Pmdieri, G. J. Organomet. Chem 19%. 308.195-206. 5. Buss, A-D.; Warren, S. J. Chem Sot. Perkin Trawl, 19W. 2307 and references therein. 6. a) Burger, L.L. J. Chem Phys. 1958, 62, 590-593. b) Rozen, A.N.; Niolotova, Z.I.; Kattasheva, N.A. Radiokhinrya, 1983,25,609-6 13. 7. Morrison, D.C. .I. Amer. Chem Sot., 1950,72,4820-4821. 8. Berlin, K.D.; Pagilagan, RU. J. Org. Chem 1%7,32, 129- 133. 9. Korpium. 0.; Lewis, R.A.; Chickos, J.; Mislow, K. J. Amer. Chem Sot.. 1968,90,4842-4846. 10.(a) Andersen, K.K. Stereochemistry, conformation, and chiioptical properties of sulfoxides. In 77te Chemistry of Sulphones and Sulphoxides; Patai, S.; Rappoport. Z.; Stirling, C.J.M. Eds.; John Wiley and Sons, Inc: New York, 1988; pp. 55-94. (b) Drabowicz, J.; KieIbasinski, P.; Mikolajczyk, M. Synthesis of sulphoxides. ibid. pp. 233-378. 1l.a) Uchida, Y.; Gnoue, K; Tada, N, Nagao, F.; Gae, S. Tetrahedron Len. 19@9,30,567-570. b) Furukawa, N.; Ogawa, S.; Matsumura, K.; Fujihara, H. J. Org. Chem 1991,56,6341-6348. 12.a) Oae, S.; Kawai, T.; Futiwa, N.; Iwasaki, F. J. Ckm Sot. Perkin Trans. 2 1987.405-411. b) Oae, S.; Takeda, T.; Wakabayashi, S.; Iwasaki, F.; Yamazaki, N.; Katsube, Y. ibid 1990.273-276 and references therein. 13Lewis. RA.; Mislow. K. J. Amer. Chem Sot.. 1969,91.7009-7012. 14.For a mechanistic study of the reactions between alkenyl halides and cuprates see: Maffeo. C.V.; Marchese. Cl.; Naso, F.; Ronzini, L. J. Chem Sot., Perkin Trans. I 1979.92-97. 15.For nucleophilic vinylic substitution with leaving and activating groups attached to the same carbon atom, see Rappoport, Z. Nucleophilic Vinylic Substitution. In Advances in Physical Organic Chemistry 7; Gold, V. Ed.; Academic Press: London 1969; pp. l-l 14. IdMaier, L. Chem Ber. l%l, 94.3043-3050.
(Received in UK 10 March 1993)
Letren. Vol. 34, No. 19. Pp. 31353138.1993
Priitcd in Grut Britain
Reactions of (l-ChlorovinyQdiphenylphosphine
Oxide
with Organometallic Reagents codimu car&nicdl&P,
kanccaco NH’, WihkWkP
Vito Fim,
F+l&ddd*,
K. Mkhd
and witdd
(‘) C.N.R., Centro di Stualiosulk Metodogie
Innovative di Sintesi Organiche. Dipartimento di Chimica, via Amendola 173,70126 Bari, Italy.
(b) Polish Academy of Science, Centre of hfokcular and Macromokcular Studies, 90-363 LU?, Sienkiewicza 112. Poland
We have recently reported the results of our studiis on the reactions of halovinyl aryl sulfoxides with organocuprates, organomagnesium
and organolithium compounds’~. In particularl. @‘,I-and Q-2-halovinyl
phenyl sulfoxides were found to tcact with alkyl- or phenylmagnesium bromides at room tempt?-,
leoding
to alkyl phenyl or diphenyl sulfoxides in good isolated yields (79-935). and a similar behaviour was found for 1-btomovinyl ~ulfoxides,~ according to Scheme 1. SCHEME 1 0
0
II
RMgBr
Ar-S-CH=CH-X
b
0
II ArS-R
4
THF
RMgBr
II Ar-S-C(Br)=CH2
THF R= alkyl. aryl
X= Cl, Br Optically active I-bromovinyl
and 2-btomovinyl
aryl sulfoxides ma&d
Grignard reagents (e.e.Z98%).2 Compkte inversion of configuration was establii
enantiospecifically
with
for the reaction with the
fast type of substrate. This stereochemical evidence is consistent with the hypothesis of an attack of the Grignard reagent at the sulphur atom of the halovinyl sulfoxide with a concomitant expulsion of a two c&on fragment, which then decomposes to give acetylene.’ These results
prompted
(1chlorovinyl)diphenylphosphii
us to investigate
the mactions
between
organometallic
teagents
and
oxide l? in order to compam the toactivity pattern between phosphorous 3135
3136
and sulphur compounds and to verify the possibility of a new synthetic approach to phosphine oxides, a class of compounds of special interest due to their versatile use: e.g. as complexing agentsP in the synthesis of natural producti and in applied chemistry.6 Diphenylphosphinyl chlorides’ and alkyl diphenylphosphinate# displacement
with
organomagnesium
reagents,
yielding
were reported to undetgo a nucleophilic
alkyldiphenylphosphine
oxides.
The
high
enantiospecificity of this process was applied to the synthesis of optically active phosphine oxides, starting from menthyl phosphinates,9 as a counterpart of the synthesis of optically active sulfoxides starting from menthyl sulfmate.tO Furthermore, it has been recently reported l1 that some phosphine oxides react with organolithium or Grignard reagents with a ligand exchange or dispropottionation, as it was reported for the analogous sulfoxides.‘2 We wish to report now that when (l-chlorovinyl)diphenylphosphine
oxide 1 was nxcted with Grignad
reagents, an attack of the organometallic compound at the phosphorous atom with consequent formation of phosphine oxides 210 was observed (see Table). Table. Reactions of (l-Chlorovinyl)diphenylphosphiie
Oxide 1 with Grignard Reagents
RMgBr
1 Ph2 P -R
Ph 2 ! - C(Cl)=CH 2’ THF, 25“C
1 bay : : : 7 f
Product
R Meb Et n-Pr c n-Bu c Ph z:g; p-Aniiyl 2-Np
210 Yield * 75
: z : 6
;:
s7 9 10
;: 78 38
a) Yields refer to pure isolated products; b) methylmagnesium chloride was used, c) reactions performed at CPCdid not show any signitlcant diierence on the products. As reported in the Table, good yields wete obtained when 1 reacted tith methylmagnesium chloride and with some aryl Grignard reagents (entries 1.5.6
and 8). If other alkylmagnesium reagents were used (entries
2-4). lower isolated yields of 3-5 wete obtained, due to the formation of minor quantities of several by-products, presumably originated by metalationS and further reaction of the alkyldiphenylphosphine
oxide
55 obtained in the fiit displacement reaction. Moderate yields were also obtained with o-tolylmagnesium and 2-naphthylmagnesium
bromide (entries 7
and 9). probably owing to steric masons. A similar argument was used to explain some unsuccessful reactions of hindered Grignard magenta with menthyl phosphinates.s9*13
3137
Gn the whole, the observations made with the Grignard reagents indicate a close analogy with the behaviour of the sulfoxides. The presented results confirm clearly the quality of the chlorovinyl group as a viable leaving group in heteroatom chemistry. It seems likely that, as in the sulfoxides, also in this case the fate of the two-carbon fragment is the decomposition to acetylene. A more complex pattern was observed when (1-chlorovinyl)diphenylphosphine cuprates in THP at 0°C (Scheme). Using a 1.51 cupmte:substrate
oxide 1 reacted with
ratio. compound
consumed. Among the examined organocuprates, n-Bu&uMgBr and Ph#tMgBr
1 was completely
gave the phosphine oxides
13 (isolated yield 49%) and 14 (yield 63%) as the main isolated products. Gas-chromatography/Mass Spectrometry analysis of both reaction mixtutes revealed minor quantities of the vinylphosphine oxides 11 tR= n-Bu. 35% of the tea&on ptoducts), 12 (R= Ph. 5%) and diphenylvinylphosphine
oxide 15 (less than 5% in
both reactions). Gn the other hand, n-BuaCuLi and n-Bu2Cu(CN)Liz caused mainly reduction of the substrate (i.e. formation of 15). while Ph&uLi gave the same products of Ph&uMgBr, although in lower yields (14 was
isolated in 47% yield) due to the formation of a greater amount of side-products.
Although a deeper
investigation would be necessary for the rationale of these observations, the formation of the above products could be tentatively explained according to the following Scheme.t4 SCHEME 2 0 II -
+
Ph+‘-C(Cl)CH2R
a-
0
II
PhzPCH=CH-R
(a)
1112
1) R&uM i Ph~PCH,CHR, 2) H+ W-14
R2CuM Ph2 P-C(Cl)=CH,
P G CUM II I Ph2 P-C=CH,
1 iI
-b
H+
00
ii
Ph, P-CH=CH, 16
11,13 M=MgBr
R = n-Bu
12,14 R=Ph
(a) The main pathway should be represented by an addition process leading to a species from which products 11-12 am originated.15 These undergo a further addition of cuprate to give 13 or 14. (b) The formation of vinyldiphenylphosphine
oxide 15 should be the result of a metal-halogen exchange followed by the final
neutralixation. In conclusion. the results of the present investigation suggest that synthetically useful teactions can be performed
reacting
I-halovinylphosphine
oxides with organometallic
reagents. Carbon-phosphorous
or
carbon-carbon bond can be formed depending upon the nature of the reagents, thus permitting to obtain different types of phosphine oxides from a common precursor.
3138
Rcactbn of (l-ChbrovinyI)diphenylphoephine Oxide 1 with Grignud Reagents, Synthesis of DIphenyhncthylp oxhk 2. A solution of 2.2 mmol of methyImagn&un chloride in THF was added dropwise to a stirred solution of 0.3 g (1.1 mmol) of 1 in 12 mL of THF, under Nz. After 1 h, the reaction mixture was quenched with a saturated aqueous NH&l solution and extracted three times with ethyl acetate. The combined organic extracts were washed with an aqueous solution of NaOH 1 N and with water, and dried (Na2SOd). Then the solvent was removed in vucuu. The residue was eluted through a very short column of basic aluminum oxide (eluent ethyl acetate). After vacuum distillation of the solvent, the residue was purified by crystallization (benzene@troleum ether, mp 109-l IOOC,lit.16 111.5-l 12 “c). Yield 75%. Rerction of (l-ChIorov@I~phenylpbo6phine Oxide 1 w&h Organocuprates. Synthesis of (2&dIphenykthylPiphenylpbasphk Oxide 14. A solution of 3.3 mmol of freshly prepamd phenylmagnesium bromide in ‘H-IF was added dropwise to a stirred slurry of 0.314 g (1.65 mmol) of CuI in 5 mL of THF at Ooc. After 10 min. a solution of 0.3 g (1.1 mmol) of 1 in 6 mL of THF was added dropwise. The reaction mixture was stirred at Ooc for 1 h and then was quenched with a saturated aqueous NH&l solution and extracted three times with ethyl acetate. The combined organic extracts were washed with water, dried (Na+O,) and the solvent was removed in VUCIW. (2,2-Diphenylethyl)diphenylphosphine oxide 14 was purified by crystallization (ethyl acetate; mp 221-222 “c). Yield 63%. ‘H NMR (500 MHz) 8 7.99-7.95 (m, 2 H), 7.58-7.03 (m, 16 H), 6.81-6.79 (m, 2 H), 3.64 (ddd. 5=3.5,7.6, 11.0 Hz, 1 H), 3.32 (ddd, .J= 5.7, 11.0, 14.1 Hz_ 1 H), 3.26 (ddd, 5=3.5,9.0, 14.1 Hz. 1 H). MS (70 ev): m/e (relative intensity) 382 (M.4). 202 (lOO), 180 (16), 165 (12). AckmMedgemcnt: Work supported in part by Miteto dell’Universita e della Ricerca Scientifica e Tecnologica, Rome and by the State Committee for Scientific Research, Poland. We thank dr. C. Bortone. dr. M.A. Prencipe and Mr. M. Koprowski for preliminary experiments. REFERENCES 1. Cardellicchio. C.; Fiidanese, V.; Naso, F. J. Org. Chem 1992,57,1718-1722. 2. CardelIicchio. C.; FianU, V.; Naso. F.; Scilimati, A. Terrahcdron Lerr.. 1992.57.5121-5124. 3. Pietrusiewicz, K.M.; Wisniewski, W.; Zablocka, M.; Tetruhcdron. 1989,#5,337-348 and references therein. 4. Dondi. S; Nardelli, M.; Pelizzi, C ; Pelizzi. G; Pmdieri, G. J. Organomet. Chem 19%. 308.195-206. 5. Buss, A-D.; Warren, S. J. Chem Sot. Perkin Trawl, 19W. 2307 and references therein. 6. a) Burger, L.L. J. Chem Phys. 1958, 62, 590-593. b) Rozen, A.N.; Niolotova, Z.I.; Kattasheva, N.A. Radiokhinrya, 1983,25,609-6 13. 7. Morrison, D.C. .I. Amer. Chem Sot., 1950,72,4820-4821. 8. Berlin, K.D.; Pagilagan, RU. J. Org. Chem 1%7,32, 129- 133. 9. Korpium. 0.; Lewis, R.A.; Chickos, J.; Mislow, K. J. Amer. Chem Sot.. 1968,90,4842-4846. 10.(a) Andersen, K.K. Stereochemistry, conformation, and chiioptical properties of sulfoxides. In 77te Chemistry of Sulphones and Sulphoxides; Patai, S.; Rappoport. Z.; Stirling, C.J.M. Eds.; John Wiley and Sons, Inc: New York, 1988; pp. 55-94. (b) Drabowicz, J.; KieIbasinski, P.; Mikolajczyk, M. Synthesis of sulphoxides. ibid. pp. 233-378. 1l.a) Uchida, Y.; Gnoue, K; Tada, N, Nagao, F.; Gae, S. Tetrahedron Len. 19@9,30,567-570. b) Furukawa, N.; Ogawa, S.; Matsumura, K.; Fujihara, H. J. Org. Chem 1991,56,6341-6348. 12.a) Oae, S.; Kawai, T.; Futiwa, N.; Iwasaki, F. J. Ckm Sot. Perkin Trans. 2 1987.405-411. b) Oae, S.; Takeda, T.; Wakabayashi, S.; Iwasaki, F.; Yamazaki, N.; Katsube, Y. ibid 1990.273-276 and references therein. 13Lewis. RA.; Mislow. K. J. Amer. Chem Sot.. 1969,91.7009-7012. 14.For a mechanistic study of the reactions between alkenyl halides and cuprates see: Maffeo. C.V.; Marchese. Cl.; Naso, F.; Ronzini, L. J. Chem Sot., Perkin Trans. I 1979.92-97. 15.For nucleophilic vinylic substitution with leaving and activating groups attached to the same carbon atom, see Rappoport, Z. Nucleophilic Vinylic Substitution. In Advances in Physical Organic Chemistry 7; Gold, V. Ed.; Academic Press: London 1969; pp. l-l 14. IdMaier, L. Chem Ber. l%l, 94.3043-3050.
(Received in UK 10 March 1993)